1. Field of the Invention
The present invention relates to image projection systems. More specifically, the present invention relates to prepolarizers for liquid crystal light valve image projection systems.
While the invention is described herein with reference to an illustrative embodiment for a particular application, the invention is not limited thereto. Those of ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope of the invention.
2. Description of the Related Art
The development of the liquid crystal light valve has opened the door to substantial progress in the state of the art of high quality large screen projectors. The reflective mode liquid crystal light valve is a thin film, multilayer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photoresponsive layer sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube is applied to the photoresponsive layer thereby switching the electric field across the electrodes from the photoresponsive layer onto the liquid crystal layer to activate the liquid crystal. Linearly polarized projection light passing through the liquid crystal layer and reflecting from the dielectric mirrors is polarization-modulated in accordance with the information incident on the photoconductor. Therefore, when a complex distribution of light, for example, a high resolution input image, is focused onto the photoconductor surface, the device converts the image into a replica which can be projected with magnification to produce a high brightness image on a viewing screen. U.S. Pat. No. 4,019,807 issued to D. D. Boswell et al on Apr. 26, 1977 disclosed such a high performance reflective mode liquid crystal light valve.
A graphics display projector using a liquid crystal light valve of the above-type is described in an article entitled "Application of the Liquid Crystal Light Valve to a Large Screen Graphics Display", published in the 1979 Society for Information Display (SID), International Symposium, Digest of Technical Papers, May 1979, pp. 22-23.
More sophisticated liquid crystal light valve image projection systems are illustrated in the following patents: U.S. Pat. No. 4,425,028, issued to R. J. Gagnon et al on Jan. 10, 1984; U.S. Pat. No. 4,544,237, issued to R. J. Gagnon on Oct. 1, 1985; U.S. Pat. No. 4,461,542, to R. J. Gagnon on July 24, 1984; and U.S. Pat. No. 4,464,019, issued to R. J. Gagnon on Aug. 7, 1984.
These designs provide high resolution, high contrast color images by separating an input beam into different optical paths for polarization and/or color processing. The beams are then recombined at or prior to illumination of a main polarizing prism. Some of the prior systems are monochrome, others utilize two colors, while some provide a full color output. The advantages of color are well recognized. Unfortunately, a full color system is a most demanding, and hence typically most costly, application for a liquid crystal light valve image projection system. That is, although the above noted full color systems effectively and efficiently provide high quality color images, the optical components required for the separation and recombination of beams adds significantly to the cost of the system. Thus, a need has been recognized in the art for less costly liquid crystal light valve color projector designs. In addition, it would be desirable to add a multi-color capability to many of the monochrome projectors which are currently in use.
A four color system holds the promise of satisfying the need in the art for a less costly liquid crystal light valve color projector design. As a less demanding application for a projection system, it would ease color requirements and require a simpler illumination system. What would remain then would be a need for a system that would allow for a retrofit of existing monochrome projectors to provide limited multi-color capability.